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[SMP43-01] Possible polymetamorphism and brine infiltration recorded in the garnet-sillimanite gneiss, Skallevikshalsen, Lützow-Holm Complex, East Antarctica
Keywords:brine, partial melting, continental collision, monazite, zircon, polymetamorphism
The core of garnet porphyroblasts in the garnet-sillimanite gneiss from Skallevikshalsen, Lützow-Holm Complex, East Antarctica, includes Cl-rich (>0.3wt%Cl) biotite and nanogranite/felsite inclusions (former granitic melt). These are estimated to be stable at >1.2 GPa and 820-850 oC. Rare occurrence of matrix biotite suggests almost complete consumption of pre-existed matrix biotite during prograde to peak metamorphism. Brine infiltration during prograde to peak metamorphism is supported by Cl-rich scapolite described in previous studies [e.g., 1]. Brine infiltration and progress of continuous biotite-consuming melting reactions were probably responsible for elevating the Cl content of biotite.
In situ electron microprobe U-Th-Pb dating of monazite and the in situ LA-ICPMS U-Pb dating of zircon in the garnet-sillimanite gneiss revealed that both monazite and zircon has the ‘older age population’ with ca. 650-580 Ma and the ‘younger age population’ with ca. 560-500 Ma. The REE and trace element pattern of one of the P-rich patches in the garnet core is different from the P-rich garnet rim. The isotope mapping of the same patch by LA-ICPMS revealed that the patch is also observed as a domain depleted in 51V, 89Y, 165Ho, 166Er, 169Tm, 172Yb, and 175Lu. Clear difference in 51V concentration between the patch and the rim of the garnet suggests that this patch is not a continuous part from the garnet rim, but is likely a relic of preexisted garnet. Kyanite included in the patch suggests that medium- to high-pressure type metamorphic rock was the precursor. Presence of the older age population (ca. 650-580 Ma) monazites in Skallevikshalsen and Skallen [2] also suggest that rocks in these areas experienced polymetamorphism, and resetting by the ca. 560-500 Ma metamorphic event was incomplete. Taking into account the presence of Cl-rich biotite inclusions in garnet, infiltration of brine accompanied by partial melting is one probable event that took place at ca. 560-500 Ma in the Skallevikshalsen area, and part of the monazite possibly recrystallized by this brine infiltration.
References: [1] Satish-Kumar et al., 2006, JMG. [2] Hokada and Motoyoshi, 2006, Polar Geosci.
In situ electron microprobe U-Th-Pb dating of monazite and the in situ LA-ICPMS U-Pb dating of zircon in the garnet-sillimanite gneiss revealed that both monazite and zircon has the ‘older age population’ with ca. 650-580 Ma and the ‘younger age population’ with ca. 560-500 Ma. The REE and trace element pattern of one of the P-rich patches in the garnet core is different from the P-rich garnet rim. The isotope mapping of the same patch by LA-ICPMS revealed that the patch is also observed as a domain depleted in 51V, 89Y, 165Ho, 166Er, 169Tm, 172Yb, and 175Lu. Clear difference in 51V concentration between the patch and the rim of the garnet suggests that this patch is not a continuous part from the garnet rim, but is likely a relic of preexisted garnet. Kyanite included in the patch suggests that medium- to high-pressure type metamorphic rock was the precursor. Presence of the older age population (ca. 650-580 Ma) monazites in Skallevikshalsen and Skallen [2] also suggest that rocks in these areas experienced polymetamorphism, and resetting by the ca. 560-500 Ma metamorphic event was incomplete. Taking into account the presence of Cl-rich biotite inclusions in garnet, infiltration of brine accompanied by partial melting is one probable event that took place at ca. 560-500 Ma in the Skallevikshalsen area, and part of the monazite possibly recrystallized by this brine infiltration.
References: [1] Satish-Kumar et al., 2006, JMG. [2] Hokada and Motoyoshi, 2006, Polar Geosci.